What was found
(CAN) AIRCRAFT IS CURRENTLY ON THAI REGISTRY BUT WE HAVE SUBMITTED THIS SDR AS THIS IS THE SECOND OCCURRENCE OF THIS TYPE IN THE PAST 2 YEARS. A/C RETURNED TO BASE REPORTING HYDRAULIC SYSTEM LIGHT. INVESTIGATION REVEALED TGB (OVERFILLED) IT WAS CONCLUDED THAT TAIL ROTOR SERVO HAD MASSIVE INTERNAL LEAK THAT CONSEQUENTLY FILLED UP TGB. TAIL ROTOR SERVO WAS REPLACED. QUICK RIGGING CHECK WAS CARRIED OUT AIRCRAFT WAS TEST FLOWN SATISFACTORY AND RETURNED TO SERVICE.